This invention relates to a process for the recovery and purification of 3,4-epoxy-1-butene (epoxybutene). More specifically, the present invention relates to the separation by extractive distillation of epoxybutene from crude mixtures with close-boiling, pinched, or azeotrope-forming compounds. The invention disclosed herein is of particular interest and utility in the recovery and purification of epoxybutene from mixtures comprising aliphatic and aromatic hydrocarbons containing five to seven carbon atoms having boiling points between about 20xc2x0 C. and 11 5xc2x0 C.
U.S. Pat. Nos. 4,897,498, 4,950,773, and 5,618,954 disclose gas phase epoxidation processes for the production of epoxybutene from molecular oxygen and 1,3-butadiene using silver-based catalysts. U.S. Pat. No. 5,362,890 describes a gas-phase epoxidation process for the production of epoxybutene in which a C2 to C6 paraffinic hydrocarbon is used as gas-phase reaction diluent in order to improve heat transfer and increase the safe oxygen levels in the epoxidation reactor. These patents further disclose that liquid 1,3-butadiene and liquid n-butane/1,3-butadiene mixtures are particularly favorable solvents for the absorptive recovery of epoxybutene from the gaseous effluent of the epoxidation reactor. An epoxide-rich absorbent obtained from the bottom of the absorption column typically comprises 1 to 40 mole percent epoxybutene, 1 to 10 mole percent water, and 30 to 98 mole percent n-butane and 1,3-butadiene. This epoxide-rich absorbent is subjected to further processing steps, i.e., decantation and distillations in order to recover substantially pure epoxybutene and n-butane and 1,3-butadiene that is substantially free of epoxybutene, i.e., less than 5000 ppm by volume of epoxybutene. Since n-butane and 1,3-butadiene have normal boiling points of 0xc2x0 C. and xe2x88x922xc2x0 C., respectively, and do not form close-boiling, pinched, or azeotropic mixtures with epoxybutene, the distillation separations disclosed therein can be accomplished in simple single feed rectification columns.
U.S. Pat. No. 5,945,550 discloses a gas-phase epoxidation process for the production of epoxybutene in which one or more C4 to C10 paraffinic hydrocarbons having high autoiginition temperatures are used as gas-phase reaction diluents in order to improve heat transfer, increase the safe oxygen levels, and substantially increase space-time yield in the epoxidation reactor. Although clearly advantageous for improving reactor performance, many of such C5 to C7 alkane diluents having boiling points between about 20xc2x0 C. and about 115xc2x0 C. form close-boiling, pinched, or azeotropic mixtures with epoxybutene. Such mixtures are not amenable to production of epoxybutene in high purity and high recovery in simple single-feed rectification columns.
Furthermore, many alkyl and aryl hydrocarbons, with boiling points between about 20xc2x0 C. and about 115xc2x0 C., are useful as absorbents for the recovery of epoxybutene from gas-phase reactor effluents, as extractants of epoxybutene from aqueous streams, and as reaction media for further derivatization of epoxybutene. Examples of such alkyl and aryl hydro-carbons include but are not limited to benzene, toluene, isopentane, n-pentane, n-hexane, n-heptane, and 2,2-dimethylbutane. These compounds also form close-boiling, pinched, or azeotropic mixtures with epoxybutene and thus cannot be separated effectively in simple single-feed rectification columns. Thus there is a need for a process to effectively separate such compounds from epoxybutene.
Relative volatility, xcex1, is defined as the ratio of the equilibrium vapor and liquid compositions of the two components to be separated. Thus,                     α        =                                            y              1                                      x              1                                                          y              2                                      x              2                                                          (        1        )            
where yi is the mole fraction of component i in the vapor phase and xi is the mole fraction of the component i in the liquid phase. The normal convention in the art is to define the lower boiling pure component as component 1 and the higher boiling pure component as component 2.
In an azeotropic system, the relative volatility will vary from greater than unity to less than unity as one passes through the azeotropic composition. At mole fractions of the lower boiling component less than the azeotropic composition the relative volatility is greater than unity, while at mole fractions greater than the azeotropic composition the relative volatility is less than unity. At the azeotropic composition, the relative volatility of the components forming the azeotrope is unity. In other words, the vapor and liquid compositions are identical. Since distillation works by differences in vapor and liquid compositions, no further separation is possible by simple distillation once the azeotropic composition is reached, even with an infinite number of equilibrium stages.
In a pinched or close-boiling binary system that shows positive deviations from ideal liquid behavior, the relative volatility will vary from greater than unity at low concentrations of the lower boiling component to close to unity at high concentrations of the lower boiling component. Thus, near the pinch point, the composition of the vapor and liquid phases approach each other, and a large number of equilibrium stages is required for further separation.
Table I shows the effect of relative volatility (xcex1) on theoretical stage requirements in terms of the number of theoretical equilibrium stages required at total reflux for the given degree of separation or purity. In Table I, Separation Purity refers to the mole fraction separation purity of both products.
Rarely is it economical or practical to operate a distillation column with more than about 60 theoretical (typically 70 to 120 actual stages). Thus, for situations where both high purity and high recovery are required, the relative volatility should be greater than about 1.2.
Extractive distillation is a method of separating close boiling, azeotropic, or pinched compounds from each other by conducting the distillation in a two-feed, multi-staged, rectification column in the presence of an added liquid or liquid mixture, said liquid(s) having a boiling point higher than the compounds being separated. The extractive agent or solvent is introduced near the top of the column, above the primary feed stage where the components to be separated are introduced. Since the extractive agent is chosen to be higher boiling than the components to be separated, the agent remains largely in the liquid phase throughout the sections of the column below the stage upon which it is fed. Extractive distillation operates by the exploitation of the selective solvent-induced enhancements or moderations of the liquid-phase nonidealities of the components to be separated. The solvent selectively alters the activity coefficients of the components being separated, thus making a greater degree of separation possible than in the absence of the extractive agent. At the bottom of the extractive distillation column, the less volatile component in the presence of the selected solvent and the extractive distillation solvent itself are removed continuously from the column. The usual methods of separation of these two components are by a second single feed distillation, cooling and phase separation, or solvent extraction. Although the principles of extractive distillation are well-known in the art, there are no a piori methods of determining the efficacy of extractive agents for a given separation, even for those ordinarily skilled in the art.
The usual method of evaluating the efficacy of extractive distillation agents is to measure the change in relative volatility of the compounds to be separated in the absence and presence of the extractive distillation agent. A practical method of measuring relative volatility is with an equilibrium still. A given composition of components 1 and 2 is charged to the still pot, heated to the boiling point, and allowed to come to vapor-liquid equilibrium. Samples of the vapor and liquid are taken, analyzed to determine the composition, and these measurements are used to calculate the relative volatility. To test the efficacy of a solvent, the experiment is repeated with solvent added to the still pot as well as a mixture of the two components to be separated. Each potential extractive distillation agent is evaluated at a constant component 1 to component 2 weight ratio and constant solvent to component 1/component 2 weight ratio.
The present invention provides an extractive distillation process or method that enhances the relative volatility between epoxybutene and aliphatic and aromatic hydrocarbons containing five to seven carbon atoms and having boiling points between about 20 and 115xc2x0 C. to improve the rectification efficiency over that of a single-feed distillation column or reactive distillation column. The process of the invention utilizes organic compounds that are stable, and can be separated from epoxybutene and the hydrocarbon(s) and recycled to the extractive distillation column with little decomposition. In its broader aspects, the present invention provides a process for the separation of epoxybutene from a mixture comprising epoxybutene and at least one aliphatic or aromatic hydrocarbon containing five to seven carbon atoms and having a boiling point of between about 20 and 115xc2x0 C. which comprises the steps of:
(1) feeding a mixture comprising epoxybutene and at least one hydrocarbon selected from aliphatic or aromatic hydrocarbons containing five to seven carbon atoms and having boiling points of between about 20 and 115xc2x0 C., to the mid-section of an extractive distillation column;
(2) feeding an extractive distillation solvent to the upper section of the extractive distillation column;
(3) removing from the upper section or top of the extractive distillation column a vapor comprising the hydrocarbon;
(4) removing from the lower section or base of the extractive distillation column a liquid comprising epoxybutene and extractive distillation solvent; and
(5) separating the liquid of step (4) into epoxybutene and extractive distillation solvent; wherein the extractive distillation solvent (i) is inert, i.e. is non-reactive with respect to epoxybutene and the hydrocarbon, (ii) does not form an azeotrope with either epoxybutene or the hydrocarbon, (iii) is miscible with epoxybutene and the hydrocarbon, (iv) has a boiling point at least about 20xc2x0 C. higher than the higher boiling point of epoxybutene and the hydrocarbon, and (v) creates a large relative volatility difference between epoxybutene and the hydrocarbon, i.e., in mixtures of epoxybutene, the hydrocarbon and the extractive distillation solvent.
In a preferred embodiment of the invention, the separation of step (5) comprises the steps of:
(5) feeding the liquid comprising epoxybutene and the extractive distillation solvent removed from the lower section or bottom of the distillation column in step (4) to the mid-section of a solvent recovery distillation column;
(6) removing from the upper section or top of the solvent recovery distillation column a vaporous distillate product comprising greater than about 99.5 weight percent epoxybutene; and
(7) removing from the lower section or bottom of the solvent recovery distillation column a liquid comprising the extractive distillation solvent.